Plasmonic devices integrate the capability of localizing and guiding light in sub-wavelength metallic structures, driving photonics and electronics towards practical applications. Therefore, these devices have generated significant interest due to their broad relevance as waveguide, filter, and light source technologies, among others.  Towards these ends, effective translation of plasmonic devices is based upon the requisite fruition of active switching and modulator components. As such, exciting work by Huang and colleagues has advanced this domain towards this goal by developing a novel photo-responsive liquid crystal/gold nanodisk hybrid as a light-driven plasmonic switch. The switches harnessed the advantages of liquid crystals due to their favorable biorefringence properties which (via photo-irradiation) were capable of modulating the localized surface plasmon resonance (LSPR) of the embedded array of gold nanodisks. This was due to the fact that LSPR is dependent upon the refractive index of the surrounding medium. Compared to existing studies that have explored electro-active plasmonic switches, this work has realized improvements in the form of a potentially faster operating speed. Furthermore, light can be applied towards writing and reading functionality towards downstream all-optical plasmonic circuit fabrication.

The elegantly designed switch architecture was based upon a gold nanodisk array and bare glass slide sandwiching the photo-responsive liquid crystal element. A 420 nm violet laser diode was used as the light source. The gold nanodisks possessed a mean diameter of 140 ± 14 nm and a period of 320 ± 32 nm.  Nanodisk production was performed via nanosphere lithography integrated with two-step reactive ion etching (RIE) techniques. In order to manipulate the surrounding medium of the integrated nanodisk arrays, photo-induced conformational changes in the liquid crystals were employed. The liquid crystal material was comprised of a mixture of 70% nematic liquid crystals, 20% 4-butyl-4’-methyl-azobenzene, and 10% chiral dopant. Azobenzene-induced cis-trans photoisomerization and resultant orientation shifting of the liquid crystals resulted in a refractive index change surrounding the nanodisk array towards LSPR modulation which drove the activity of the light-activated switch. Trials examining the activity of the liquid crystal/nanodisk hybrid devices demonstrated changes in the extinction spectra by altering the incident angles of the light source with and without the applied 420 nm violet laser diode light source (20 mW). Changes in blue shift properties (from 30 nm to 10 nm) were observed when the incident angle of the light was changed from a normal position to the substrate to  50 degrees with respect to the normal direction of the substrate. This confirmed a preferential liquid crystal orientation following photo-activation. Tunable/reversible switching was also observed by turning the light source on and off which resulted in response times of 17.1 s and 1.5 s, respectively. Furthermore, because the light shuttering was accomplished via manual blockage, the intrinsic switching speed of the processes inherent to this device are expected to be further understood and optimized with subsequent studies. Continued work will seek to examine how the manipulation of several parameters such as azobenzene content, light power, substrate topology, among others, can be tuned to influence switching response times.

This work successfully developed an all-optical switch with reversible LSPR tuning capabilities. Azobenzene photoisomerization-driven orientation changes in the liquid crystals were demonstrated as the mechanisms for refractive index manipulation surrounding the gold nanodisks. Plasmonic circuits, optical storage technologies, as well as potential biological integration applications (where certain biological molecules are also capable of undergoing photo-dependent conformation changes and surface plasmon resonance has played an important role in diagnostics and monitoring of binding/unbinding reactions) are all examples of domains that may be impacted by continued material-based advances in device performance metrics (e.g., response time, robustness, etc.). Advantages introduced by examples including the work by Huang and colleagues may realize new tools that are applicable towards several fields of research.